The present invention relates to an apparatus and a method for laser processing and, more particularly, relates to an apparatus and a method for performing laser processing by projecting an image of a mask, in which an aperture in An arbitrary pattern is formed, on a workpiece.
A conventional technology related to a laser processing apparatus will be described with reference to a drawing.
FIG. 7 is a schematic diagram showing a configuration of a conventional laser processing apparatus. In FIG. 7, a laser beam 72 emitted from a laser oscillator 71 is passed through a lens system 73 and thrown onto a mask 74. Mask 74 contains an aperture whose opening is variable or fixed. There are cases where the opening of the aperture in the mask is varied according to the size of the hole to be formed in the workpiece and where masks having apertures of different openings are selectively used according to the need in each processing step. Lens system 73 is designed such that the beam diameter of laser beam 72 incident on the mask and the curvature radius of the wave surface produce optimum conditions for the processing. A projecting lens 75 projects the image of the mask onto a workpiece 76 and thereby holes are bored in a workpiece such as a printed board.
Further, such arrangements are devised in the prior art to scan a laser beam using a galvano-mirror or to speedily shift stages between steps of processing so that as many steps of processing as possible may be made in a short time.
However, conventional laser processing apparatus had problems as follows:
In performing laser processing for such work as hardening, welding, and making a hole or the like in a resin impregnated laminated substrate containing a lot of glass fibers, it is desired that the intensity distribution of the laser beam on the workpiece be uniform. However, in many cases when CO2 laser, YAG laser, or the like is used, the intensity of the laser beam is stronger in the vicinity of the optical axis and becomes weaker exponentially as it goes toward the circumference, i.e., the intensity distribution becomes close to the Gaussian distribution. When such a laser beam is used for example in boring a hole in a resin impregnated laminated substrate, long projections of fibers are produced on the inner wall of the via-hole and this causes defects of metal plating performed in a subsequent process.
Further, a conventional laser processing apparatus is designed to sufficiently magnify the laser beam with respect to the aperture in the mask using a collimator and use only the strong rays along the optical axis and in its vicinity for processing. When such a design is carried out, the energy of the laser beam blocked by the screening portion of the mask becomes large and, hence, energy efficiency is decreased.
Further, in another type of conventional laser processing apparatus, a multimode laser beam is generated, and the laser beam whose intensity distribution is thus made virtually uniform is used. However, in a multimode laser beam, the mode pattern generally varies with changes of the laser output and the intensity distribution tends to fluctuate. As a result, such a problem arises that the intensity distribution on the processed surface fluctuates and, hence, it becomes impossible to stabilize the processing performance.
The laser processing apparatus of the invention comprises a mask for limiting the transmission area of a laser beam to an arbitrary shape , a variable-magnification optical system for projecting the laser beam with a uniform intensity distribution at a predetermined position onto the mask at a magnification factor suited to the transmission area of the mask, and an optical system for projecting the mask pattern onto a workpiece. By the use of the laser processing apparatus of the invention, a uniform intensity distribution of the laser beam in compliance with the size of the mask pattern can be obtained on the workpiece and, hence, high quality laser processing can be made.
The invention further comprises a means for making the intensity distribution of the laser beam uniform at a predetermined position. The means comprises an intensity converting element for making uniform the intensity distribution of the laser beam and a phase matching element for matching the phase of the laser beam once distorted by the intensity converting element.
The invention further has, within the means for making uniform the intensity distribution of the laser beam, a lens system disposed in front of the intensity converting element for limiting variations of the beam diameter of the laser beam entering the intensity converting element.